Question

I'm securely connected to my (self-hosted) webmail.
https://webmail.whatever.com
I've tried to access the SpamAssassin Configuration option within it but I get the following.

You have asked Firefox to connect securely to mailboxes.whatever.com, but we can't confirm that your connection is secure.
mailboxes.whatever.com uses an invalid security certificate.
The certificate is only valid for the following names:

I understand why I'm getting the message, but how do I add an exception when the option to do so is not showing as it usually does?
I can try and use the non-secure URL for my webmail, but I'd rather not and I might get the same results anyway.

The part of the page that says "I understand the risks", or the Add Exception button, is completely missing? In this thread, there are references to adding an exception manually in the Options/Preferences dialog: "Add Exception" for an untrusted connection. Is that workaround relevant to your situation?

As for the solution below, there's talk of removing some user facing options that could potentially make things more challenging for you SUMO contributors. I'm sure that's a ways off, but y'all are probably the first to catch the heat when things disappear, are moved, etc.
http://limi.net/checkboxes-that-kill

Note that it is always best to check why the certificate isn't trusted to see if it is possible to solve this otherwise and that it is best to be careful whether to set a permanent exception because it is possible that the problem is only temporarily and may get fixed in the next few days and you may not notice this.

In case of missing intermediate certificates it is usually possible to install such an intermediate certificate via other sources and expired certificates because the website failed to renew can be reported to the website so they can fix this.

You should always check why there is a problem (expand the Technical Details) and be very cautious with setting a permanent exception, but check if this can be fixed otherwise.
This is quite likely an issue with the server configuration or a problem on your side (e.g. wrong date/time on the computer or security software or a wrongly saved intermediate certificate).